1. Field of the Invention
This invention relates to a process for separating ethylbenzene from a feedstream containing meta-xylene wherein a zeolite is employed as selective adsorbent.
2. Description of Related Disclosures
A class of crystalline aluminosilicates, commonly known as zeolites, constitutes well known adsorbents for use in separating certain hydrocarbon compounds from mixtures thereof. In particular, zeolites are widely used for selective separation of para-xylene from mixtures thereof with other C.sub.8 aromatic compounds such as meta- and/or ortho-xylene and/or ethylbenzene. For example, U.S. Pat. Nos. 3,636,121; 3,686,342; 3,686,343; 3,835,043; 3,855,333; 3,878,127; 3,894,108; 3,903,187 and 4,265,788 are all directed toward methods of separating para-xylene from mixtures thereof with various hydrocarbons or of selectively obtaining para-xylene and ethylbenzene from a mixture containing other components, using various types of zeolites as adsorbent. In U.S. Pat. No. 3,668,267 selective separation of 2,7-dimethylnaphthalene from a dimethylnaphthalene concentrate is effected using sodium Type Y zeolite.
While separation and recovery of para-xylene from other xylene isomers or ethyl benzene is desirable since para-xylene is an important raw material for the synthesis of terephthalic acid, it has become increasingly desirable to recover ethylbenzene selectively from feedstreams containing ethylbenzene and other xylene isomers. This is because of its commercial importance in the manufacture of styrene monomer and its increasing cost of production from reaction of benzene with ethylene. Feedstreams containing ethylbenzene and meta-xylene may be obtained as by-products from a separation of para-xylene therefrom or may be produced by a solvent extraction or fractionation process from a pyrolysis gasoline or from a naphtha reformed with a catalyst which is an oxide containing platinum and halogen.
It is know that potassium--substituted Type Y zeolites having the faujasite structure selectively adsorb ethylbenzene from mixtures comprising ethylbenzene, meta-xylene and ortho-xylene using toluene as a desorbent. U.S. Pat. No. 3,998,901 teaches that ethylbenzene can be separated from xylene isomers using a Type Z zeolite substituted with Sr and K wherein ethane or lower gases or toluene is used as desorbant. According to U.S. Pat. No. 3,943,182, desorbents other than toluene such as diethylbenzene and/or benzene selectively separate ethylbenzene from a mixture containing ethylbenzene and at least one xylene isomer using a Type X zeolite. It is also disclosed that selectivity for ethylbenzene over its isomers decreases as the silica to alumina ratio in the zeolite is increased above 3.0 (i.e., using a Type Y zeolite).
U.S. Pat. No. 3,943,182 further teaches that the presence of water in the zeolite in amounts of 0.02 to 2.5% by weight measured by loss on ignition at 500.degree. C. optimizes selectivity for ethylbenzene. Other patents disclose that certain compounds will modify the adsorbent characteristics of zeolites when contacted therewith. For example, in the context of aromatic isomer separation, U.S. Pat. No. 3,698,157 discloses that an organic radical-substituted silane modifies the characteristics of a selected zeolite in the separation of C.sub.8 aromatic isomers. In U.S. Pat. No. 3,734,974 it is taught that faster exchange rates and reduced ortho- and meta-xylene tailing are accomplished by adding small amounts of water to a particular adsorbent. Moreover, U.S. Pat. No. 3,855,333 is directed to use, as an adsorbent, of a zeolite containing 0.1 to 8.0% by weight of an alcohol to obtain increased selectivity of the zeolite for adsorption of para-xylene.